Improve handling of the "next URL" request parameter

Add proper methods to add the parameter into HTML forms or links, and use them in templates.

Fixes issue #2 (closed).

login/middleware.go

@@ -71,7 +71,7 @@ func (m *loginMiddleware) Wrap(wrap http.Handler, renderer *common.Renderer) htt
 	mux.HandleFunc(m.urls.URLFor("/login/"), func(w http.ResponseWriter, req *http.Request) {
 		var auth Auth
 		if m.authSessionStore.getSession(req, &auth) == nil && auth.check() == nil {
-			redirURL := req.FormValue("r")
+			redirURL := req.FormValue(nextURLParamName)
 			http.Redirect(w, req, redirURL, http.StatusFound)
 			return
 		}
@@ -110,7 +110,7 @@ func (m *loginMiddleware) Wrap(wrap http.Handler, renderer *common.Renderer) htt
 
 		// Redirect to login workflow.
 		values := make(url.Values)
-		values.Set("r", req.URL.String())
+		values.Set(nextURLParamName, req.URL.String())
 		http.Redirect(w, req, m.urls.URLFor("/login/?"+values.Encode()), http.StatusFound)
 	})